The need for high density connections in integrated circuit devices has led to the development of ball grid array packages which require large numbers of solder ball connections in a small area. This has typically been achieved by the robotic pick-and-place of small standard sized eutectic solder spheres onto precise locations on the package. Such a method is understandably very costly and requires specialized equipment and materials.
The present invention overcomes the difficulties of such a method by providing a solder screening method that avoids the need to precisely pick up extremely small solder balls and locate these on the package. The method enables solder balls to be formed from the paste directly on the package.
The present invention provides a method of forming solder connections on a substrate by placing a fixture over the substrate and screening solder paste onto the substrate through the fixture. The fixture is made of a material that is non-wettable by the solder and forms solder balls at the contact sites on the substrate when the solder is reflowed. Some suitable materials which are not wettable by the solder are titanium, molybdenum and graphite.
U.S. Pat. No. 4,914,814 describes a process for forming pins on a ceramic substrate carrier. A non-wettable pin mold having an array of pin holes is placed over the carrier and the pin holes are filled with high temperature melting point solder balls or solder wires. The pin mold is formed of a material having a thermal coefficient of expansion matching that of the substrate and is non-wettable by the solder. A preferred mold material in the patent is graphite. The pin mold and substrate carrier are then passed through a high temperature furnace to form the solder pin connections on the substrate. The mold is then removed and the substrate carrier assembly is then mounted on an organic circuit board with connections made through low melting temperature solder paste to pads on the board. This patent requires the use of pre-formed solder balls or wires. Among other differences, the present invention screens a solder paste through a fixture or mask. This provides more flexibility in the design as the solder ball size can be readily modified, solder paste being much easier to apply and use.
U.S. Pat. No. 5,211,328 describes a process where a transfer member is formed from graphite, ceramic or titanium with a plurality of holes located therein. The holes are precisely aligned with contact locations on a substrate. Solder is squeegeed into the holes and the transfer member placed in precise position on the substrate and the solder reflowed to form contacts at the contact sites. The transfer member is non-wettable by the solder. This patent differs in one aspect from the present invention in that the solder is squeegeed onto the transfer member and then brought into position on the substrate, whereas in the present invention, the fixture is placed on the substrate before solder is deposited into the holes in the fixture. This feature alone provides a more flexible and reliable process than the one described in the patent.
U.S. Pat. No. 5,024,372 describes a method of forming high density solder bumps on a substrate. According to this patent, solder bumps are formed by squeegeeing solder paste through a stencil or placing solder balls in precise locations on the substrate. The stencil process has limited density due to slumping of the paste when the stencil is removed. Placement of the solder balls creates a reliability problem as it is difficult to ensure that solder balls are precisely located at the appropriate locations. This patent attempts to overcome these problems by the use of a thick layer of photo-definable solder resist which is selectively removed to provide wells at solder pads on the substrate. Solder paste is squeegeed into the wells and the solder reflowed. The solder paste wets and wicks to the metalized pads on the substrate and the resist is then removed to leave solder bumps on the metalized pads. By reflowing the solder while the non-wettable fixture remains in place, the present invention overcomes the problems of slumping while providing a fixture that is readily detached from the substrate without the need of such techniques as those required in this patent. Additionally, the present invention does not require the melting point of the solder to be lower than that of the contact pad on the substrate, as is required in this patent.
U.S. Pat. No. 4,412,642 describes aprocess for casting solder leads onto a leadless chip carrier. A molding plate having a plurality of mold cavities at predetermined locations each receive a solder preform. The preform is spherical and the cavities are tapered. The plate and solder preforms are subjected to heat and pressure which is applied to the preforms to force the solder preforms into the cavities. The molding plate with the leads is then mounted adjacent a leadless chip carrier with the solder leads aligned with contact areas on the carrier. The carrier and the mold plate are heated to reflow the solder and transfer the solder to the contact areas of the carrier. The molding plate may be made of titanium to prevent wetting contact with the solder and therefore facilitate release of the solder onto the carrier. However, the process described in this patent requires the use of expensive solder preforms, as opposed to relatively less expensive solder paste. Additionally, the present invention assures that the size of the solder balls ultimately produced is controlled by the size of the apertures in the fixture. In contrast, U.S. Pat. No. 4,412,642 requires the solder preform to be determinative of the size of the connector.
U.S. Pat. No. 5,284,287 describes a vacuum tool that picks up solder balls and places these in position for soldering. The solder balls are aligned by a vacuum tool.
U.S. Pat. No. 4,712,721 describes systems for delivering preformed solder to contact sites. The preformed solder shapes are held in a positioning means which is adapted to hold the solder. The positioning means is placed in a fixture and the solder reflowed to form connections to a chip carrier package or the like. The patent does not use a stencil or template to locate the solder.
U.S. Pat. No. 3,647,533 describes a process for forming bonding bumps on a substrate array by thin film vacuum deposition through a mechanical mask. The bumps formed by the deposition are then dipped in a solder bath.
U.S. Pat. No. 5,261,593 describes a method for connecting flip chips to flexible printed circuit boards. Solder paste is placed on the contact areas on the printed circuit board and solder bumps formed on the chips are brought into registration with the contacts, this assembly then heated to reflow the solder and form contacts between the chips and flexible circuit board.
U.S. Pat. No. 5,197,655 describes a process for applying solder to fine pitch leads. In one embodiment, solder paste is screened through an apertured mask onto land locations on a printed circuit substrate. A heated platen having an active element corresponding to the size and shape of the lands is then brought into contact with the solder paste to reflow the solder and form contacts on the lands. The heated platen is made of non-wettable material such as stainless steel or titanium.
U.S. Pat. No. 5,118,027 describes a process for attaching high melting point solder balls to contacts on a substrate through the use of low melting point solder paste. The solder balls are placed in cavities in an alignment boat that holds the solder balls in place through apertures connected to a vacuum source. The solder paste is then deposited on the solder balls through a metallic contact mask. A self-aligning plate is then placed over the boat and the substrate aligned on the plate so that the contact areas on the substrate are in contact with the solder paste. Pressure is applied to the substrate to assure firm contact with the paste. The self-aligning plate is then removed and the substrate, solder and alignment boat are fed through a low temperature furnace to solder the solder balls to the substrate through the solder paste. The substrate is then available for solder joining operations where the high temperature solder ball is soldered to a board or like structure. The solder paste screen is of stainless steel, brass or copper. Stainless steel is preferred because of its wear characteristics. The alignment boat is preferably made of graphite for its thermal conductivity and coefficient of expansion compatibility with a ceramic substrate.
U.S. Pat. No. 3,458,925 describes a process for forming mounds of solder on lands on an integrated circuit chip. A mask covers the surface of the chip except for the land and the immediately surrounding area and a layer of solder is evaporated onto the lands and surrounding area. The solder is then heated to a temperature above its melting point and the solder dewets the area around the lands and forms solder mounds on the lands.
The Advanced Packaging (Summer, 1993) article by David Hattas describes a "bump grid array" package which has an array of solder bumps formed on an epoxy-fiberglass resin printed circuit board. The bumps are formed from solder paste which is deposited on the substrate through a stencil. A solder paste that apparently overcomes slumping problems is used in addition to use of a nitrogen reflow oven to attempt to overcome problems with the unwanted formation of solder balls. The article does not disclose what material is used for the stencil although one would presume that it is a non-wettable material that would withstand soldering temperatures. In contrast, the invention provides that the relative width of the opening in the fixture relative to the width of the solder pad connector can be selected such that the solder wets to the pad and does not adhere to the fixture. The invention is therefore capable of forming solder balls of predetermined size. The size of the aperture is selected such that the fixture does not contact the solder ball after solder reflow. According to this article's design, direct screening without a physical barrier is limited to a certain pitch, with resulting volume variations. In sharp contrast, the fixture of the invention provides an exact volume of paste. The article also requires a specific, customized paste to be used and is thus not adaptable to ready usage of a variety of pastes, whereas the invention is capable of using a wide variety of solder pastes. The invention also makes it possible to alter the volume of the solder balls simply by altering the thickness of the fixture or the size of the apertures in the fixture. Finally, stencils such as those described in the Hattas article are incapable of releasing high volumes of solder at a fine pitch or in a tight area.
It is believed that a method of applying solder on a substrate which overcomes the aforementioned disadvantages which assure the aforedescribed distinctive advantages, among others, would constitute a significant avancement in the art.